You can't mess anything up. Unless you completely burn your circuit board with your solder gun. Pretty hard to do. haha
Let me know if you have any questions! It was the best mod I have done or will ever do to my little Focus. I just cringe at the thought of hearing that now. I love it... just a nice pleasant red light blinking....
You can do it man. Grab an old circuit board like an old video card or network card; whatever you have and practice soldering wires to those spikes sticking up.
When you wiggle/bust off that speaker, you get a way bigger lead to solder to than you would practicing like I mentioned.
EDIT: OOPS Sorry.. I thought this was a reply to my door chime mod. hehe
Modify Your Annoying Door Chime Into LED Indicator Light: